ESOI for solar thermal

Recently, Barnhart & Benson [1] introduced a new metric to evaluate various technologies for energy storage.  They analysed seven storage technologies based on batteries, flow batteries and geologic storage, but did not consider thermal storage. 

The unanswered billion dollar question is how well do solar thermal storage technologies rate on their metric?

The Barnhart-Benson metric, Energy Stored On Invested (ESOI), is the ratio between the energy a device can store in its entire life and the energy required to build the device.

The larger the ESOI, the better is the storage system.  Larger values of ESOI can be obtained by

• increasing the number of cycles
• increasing the round-trip efficiency
• increasing the depth of discharge
• decreasing the embodied energy

Barnhart & Benson gave the following ESOI values:
 

Technology	ESOI
compressed air energy storage	240
pumped hydro storage	210
Li-ion battery	10
Sodium-Sulphur battery	6
Vanadium redox battery	3
Zinc-Bromine battery	3
Lead-acid battery	2

The conclusion by Barnhart & Benson was that

“over their entire life, electrochemical storage technologies only store 2-10 times the amount of energy that was required to build them”.

Clearly that news will not be welcomed by proponents of electrochemical storage. You can bet that feverish work is under way in hundreds of research laboratories around the world to boost the ESOI score.

Published information is available to evaluate the ESOI score for the most common solar thermal storage technology – a molten 60-40 mixture of sodium and potassium nitrates, commonly known as solar salt. 

Burkhardt, Heath and Turchi [2] made a life cycle assessment of a hypothetical 100 MW parabolic trough concentrating solar plant at Daggett, California.  The storage envisaged is 62,000 t of solar salt, capable of storing 1,988 MWh of thermal energy, which can be converted into an electrical equivalent by multiplying by the thermal-electric efficiency of the plant.

Many individual items were taken into account by Burkhardt et al. to calculate the embodied energy of the storage component of the plant; these included obvious items like steel, concrete, pumps, heat exchangers, insulation and solar salt.  However the biggest single item is the energy required to keep the salt molten and stirred for daily operations.

It’s noteworthy that the embodied energy of solar salt is low if it mined (as assumed to be the case in [2]), but high if it produced synthetically.  In the latter case, which Burkhardt et al. say applies to slightly more than half of all installations, the manufacturing process involves pre-production of ammonia, for which there is a natural gas requirement.

I have also made an as-yet unpublished estimate for the ESOI score for thermal storage in air-blown pebble beds.  This estimate is in the context of a new concept for solar thermal power generation entitled BRRIMS, denoting Brayton-cycle, Re-heated, Recuperated, Integrated, Modular and Storage-equipped.  Here what needs to be considered is the embodied energy in hardware such as steel tanks, ducts, concrete footings, insulation and pebbles.  Heat exchangers, pumps and fans are not required.

Results of Barnhart & Benson can now be extended as follows, with the new data highlighted.  This is a fair comparison (“apples with apples”) between storage technologies since the new figures represent electrical energy that would be produced from the underlying thermal storage. 

Technology	ESOI
compressed air energy storage	240
pumped hydro storage	210
pebble bed thermal, BRRIMS	62
solar salt, parabolic trough [2]	47
Li-ion battery	10
Sodium-Sulphur battery	6
Vanadium redox battery	3
Zinc-Bromine battery	3
Lead-acid battery	2

The simple conclusion from the ESOI metric is that geologic storage is excellent, thermal storage is good, whilst electrochemical storage is poor.

That is not the whole story however.  Geological storage is not particularly cheap, and its applicability is limited by the availability of suitable sites.  My estimates show that thermal storage is the cheapest option, and I propose to present details of this work at the World Renewable Energy Congress in July.

References

[1] C J Barnhart and S M Benson, “On the importance of reducing the energetic and material demands of electrical energy storage”, Energy Environ. Sci., 6 (2013), 1083.

[2] J J Burkhardt III, G A Heath and C S. Turchi, “Life cycle assessment of a parabolic trough concentrating solar power plant and the impacts of key design alternatives”, Environ. Sci. Technol. 45 (2011), 2457–2464.

 

 

Cost of solar power (34)

Usually, a student of solar power does not have access to high-quality information about projects.  Financial information is almost always lacking, reasonably enough since it’s commercially sensitive.  In other cases, project specifications are given in headline form only, direct from the Media Release, without concern for engineering accuracy.

But sometimes – rarely, once in a blue moon – all necessary information is readily available, as discussed in this post.

Three years ago, the National Renewable Energy Laboratory (NREL) published a report [1] on the design of a 100 MW parabolic trough solar thermal power station in Daggett, California.  The design includes 6 hours molten salt thermal storage.  The power block is conventional steam Rankine-cycle with wet cooling, although analysis of dry cooling was included as an option.

I should mention that this is a design exercise, not a plant that has been constructed.

NREL appointed consulting engineers, Worley Parsons, to design the plant.  And NREL is to be congratulated because the 112 page report includes extremely comprehensive details, both financial and engineering.  I recommend this report strongly if you wish to understand the ins-and-outs of solar thermal power.

In brief, the Daggett specifications envisage 100 MW_e power output.  There are 405,888 parabolic mirrors with area 987,540 m^2, 43,488 solar collector receiver tubes and 1,208 solar collector assemblies/drives.  The plant footprint is about 4 km^2, and the thermal storage of 1,988 MWh_th requires 62,000 tonnes of solar salt (a 60-40 mix of sodium and potassium nitrates).  The annual output is 426,717 MWh_e and the cost of the plant in 2010 USD is $1.016 billion.

To give an indication of the quality of the information in the report, the parasitic power losses with the wet-cooled plant are given explicitly.  The gross output of the steam turbine is 118 MW_e, the pumps to circulate the Heat Transfer Fluid (HTF, a synthetic oil) consume 7.9 MW_e, the condensers consume 2.0 MW_e and other parasitic losses are about 5 MW_e. 

As a further indication of the detailed nature of the report, here are the listed components of the thermal storage system:

• 2 cold salt tanks (including foundations)
• 4 cold tank immersion heaters
• 2 hot salt tanks (including foundations)
• 4 hot tank immersion heaters
• 3 hot salt pumps
• 3 cold salt pumps
• 6 salt to HTF heat exchangers
• 62,000 t bulk salt storage
• and a nitrogen storage and vaporisation system, especially to minimise fire risk should the HTF vaporise

Well, I could go on at length, but hopefully the point is clear.  This report is a goldmine for anyone who wants to know the constituent parts of a parabolic trough solar thermal power station.

Let me now turn to the main point of this series of blog posts – the cost of the solar power that would be produced by the Daggett plant were it to be built.  The report gives a figure for the Levelised Cost of Electricity (LCOE), namely USD 184/MWh_e “nominal”, which I take to mean before inflation.  However this figure is based on an Income Tax Credit, which is not available outside the USA, so I’m going to calculate the LCOE using my standard methodology, for which the assumptions are:

• there is no inflation,
• taxation implications are neglected,
• projects are funded entirely by debt,
• all projects have the same interest rate (8%) and payback period (25 years), which means that the required rate of capital return is 9.4%,
• all projects have the same annual maintenance and operating costs (2% of the total project cost), and
• government subsidies are neglected.

For further commentary on my LCOE methodology, see posts on Real cost of coal-fired power, LEC – the accountant’s view, Cost of solar power (10) and (especially) Yet more on LEC.  Note that I am now using annual maintenance costs of 2% rather than 3% as in posts during 2011.

The results for the Daggett design are as follows:

Cost per peak Watt              USD 8.95/Wp

LCOE                                     USD 272/MWhr

The components of the LCOE are:

Capital           {0.094 × USD 1,016×10^6}/{426,717 MWhr} = USD 224/MWhr
O&M              {0.020 × USD 1,016×10^6}/{426,717 MWhr} = USD 48/MWhr

By way of comparison, LCOE figures (in appropriate currency per MWhr) for all projects I’ve investigated are given below.  The number in brackets is the reference to the blog post, all of which appear in my index of posts with the title “Cost of solar power ([number])”:

(2)        AUD 183 (Nyngan, Australia, PV)

(3)        EUR 503 (Olmedilla, Spain, PV, 2008)
(3)        EUR 188 (Andasol I, Spain, trough, 2009)
(4)        AUD 236 (Greenough, Australia, PV)
(5)        AUD 397 (Solar Oasis, Australia, dish, 2014?)
(6)        USD 163 (Lazio, Italy, PV)
(7)        AUD 271 (Kogan Creek, Australia, CLFR pre-heat, 2012?)
(8)        USD 228 (New Mexico, CdTe thin film PV, 2011)
(9)        EUR 200 (Ibersol, Spain, trough, 2011)
(10)      USD 231 (Ivanpah, California, tower, 2013?)
(11)      CAD 409 (Stardale, Canada, PV, 2012)
(12)      USD 290 (Blythe, California, trough, 2012?)
(13)      AUD 285 (Solar Dawn, Australia, CLFR, 2013?)
(14)      AUD 263 (Moree Solar Farm, Australia, single-axis PV, 2013?)
(15)      EUR 350 (Lieberose, Germany, thin-film PV, 2009)
(16)      EUR 300 (Gemasolar, Spain, tower, 2011)
(17)      EUR 228 (Meuro, Germany, crystalline PV, 2012)
(18)      USD 204 (Crescent Dunes, USA, tower, 2013)
(19)      AUD 316 (University of Queensland, fixed PV, 2011)
(20)      EUR 241 (Ait Baha, Morocco, 1-axis solar thermal, 2012)
(21)      EUR 227 (Shivajinagar Sakri, India, PV, 2012)
(22)      JPY 36,076 (Kagoshima, Kyushu, Japan, PV, start July 2012)
(23)      AUD 249 (NEXTDC, Port Melbourne, PV, Q2 2012)
(24)      USD 319 (Maryland Solar Farm, thin-film PV, Q4 2012)
(25)      EUR 207 (GERO Solarpark, Germany, PV, May 2012)
(26)      AUD 259 (Kamberra Winery, Australia, PV, June 2012)
(27)      EUR 105 (Calera y Chozas, PV, Q4 2012)
(28)      AUD 245 (Nyngan and Broken Hill, thin film PV, end 2014?)
(29)      AUD 342 (City of Sydney, multiple sites, PV, 2012)
(30)      AUD 281 (Uterne, PV, single-axis tracking, 2011)
(31)      JPY 31,448 (Oita, PV?, Japan, to open March 2014)
(32)      USD 342 (Shams, Abu Dhabi, trough, to open early 2013)
(34)      USD 272 (Daggett, California, trough with storage, designed 2010)

Conclusion

The LCOE for the Daggett design is comparable to the figure for Blythe, see Cost of Solar Power (12).  (The Blythe plant has an interesting history; the figures used in my analysis were for a parabolic trough plant to be built by Solar Millenium.  However at a very late stage, as discussed here, the proponents switched the design from solar thermal to PV.) 

The Daggett LCOE is about 30% more expensive than the Crescent Dunes project, see Cost of Solar Power (18), which is now nearing completion.  I think this is mainly due to a general decrease in costs as we proceed along the “experience curve”, perhaps also to an advantage that heliostat/tower plants have over parabolic trough plants, notably in that parasitic losses to pump the HTF around the plant are avoided.

The other important point about the Daggett study is just how comprehensive it is.  I regard it as valuable reading for students of solar power.

Reference

 [1]  C. Turchi, “Parabolic trough reference plant for cost modeling with the solar advisor model (SAM)”, Technical Report NREL/TP-550-47605 (July 2010).

Farewell to colleagues

 

There was a terrible helicopter accident on Thursday 21 March 2013 at Bulli Tops, just south of Sydney.  The four people who lost their lives were John Dunlop, 66, Tony Farmer, 68, Gerry Haddad, 71 and Don Price, 67.  I knew them all and had a special friendship with Don.  I write this post to honour their memory.

I worked for CSIRO Australia, the national applied research laboratory, for nearly 25 years in two stints.  In the 1980s, I was located at the National Measurement Laboratory in suburban Lindfield in Sydney and it was there that I got to know the four, all of them physicists on site.  I eventually moved to another CSIRO site in 1992 and we drifted apart somewhat, although we continued to catch up occasionally.  By 2013, all four had recently retired after distinguished careers.  All leave behind loving wives, partners and families, to whom I send heartfelt condolences.

I shared many things in common with all four, especially our love for science and how it should be applied to make the world a better place. 

During the 1980s I was an enthusiastic distance runner, as was Don, and we trained together almost every lunchtime for about six years.  John was another member of the regular running group.  With Tony and Don I shared a passion for squash, a popular game at the time, and we played friendly matches together on many occasions.  With Gerry I shared participation in a CSIRO Research Leadership course and I recall many animated discussions (furious agreement really!) on any number of topics.

What do I miss especially about these friends?

Don’s considerable talents might not have been obvious to all at first because of his modesty, but he had a huge work ethic, a steely determination, an understated sense of humour and was absolutely reliable in all aspects of his life.  In addition to his scientific achievements, he was an accomplished amateur sportsman, especially in squash and running where he completed many marathons and had a best time under 2 hours 40 minutes.

I remember John’s sharp intelligence and dry wit as we would dissect the latest CSIRO management disasters on our lunchtime runs, Tony’s friendly greetings in the interminable corridors of the National Measurement Laboratory, and Gerry’s desire, eventually fulfilled, for a senior management role in CSIRO.  All were dedicated professionals, the sort of admirable colleagues you knew you could rely on through the ups and downs of a career in a government research organisation.

So, I’m grieving for all four.  The happy memories I have are a comfort to me, but the coming week will be a tough one for everyone who knew them, with multiple funerals to attend.

I especially extend my deepest sympathies to the loved ones all four have left behind.  Their loss is great.

 

Pebble bed paper accepted

The Editor of Applied Thermal Engineering this week accepted for publication my paper on pebble bed heat storage simulations.  Even at my age, now 64, I still get a buzz from the acceptance of a paper, so I thought I’d share the news here.

The title and abstract are as follows:

Simulations of air-blown thermal storage in a rock bed

This paper presents computer simulations of air-blown thermal storage in a loosely packed bed of rocks.  An important application is storage of solar thermal energy for power generation or process heating.  A new formulation is developed for one-dimensional flow of air through the rock bed, including the variation of density with temperature.  The model equations are solved numerically and results given for the effect of important parameters such as particle size, depth of bed and air flow-rate.  It is shown to be useful for the rock bed to be charged with downwards airflow and discharged with upwards airflow. This schedule is always superior – sometimes significantly so – to a schedule in which the bed is charged and discharged in the same (upwards) direction.

I began to think about these simulations in mid-2011 when I realised that thermal storage in pebble beds would be a good fit with my evaporation engine.  I designed and coded the simulation algorithm in early 2012.  In one part of the paper I compare my simulations with those of Hänchen et al. (2011) whose work also includes experimental validations.  Those comparisons show that results from my code match well with experimental results.  That’s always pleasing!

I then go on to look at the importance of molecular diffusion inside individual pebbles.  For pebbles up to about 50 mm in diameter, you might as well just assume the pebbles heat up or cool down uniformly.  Full molecular diffusion gives slightly more accurate results, but the extra accuracy hardly justifies the extra computation that is required.

Another topic I studied is the sharpness of the front between hot and cold zones in the bed.  The front becomes sharper as the particle diameter decreases. 

Yet another topic is the efficacy of charging and discharging through bi-directional (2-way) and uni-directional (1-way) strategies.  In the 2-way strategy, the bed is charged with downwards airflow and discharged with upwards airflow.  In the 1-way strategy, the airflow is always upwards.  Representative results can be viewed at www.sunoba.com.au (follow the link to “Thermal storage simulations” on the right-hand side).  My conclusion here is that the 2-way strategy is always to be preferred, even if it requires extra plumbing and ducting of the pebble bed.

The last section of my paper looks at two sorts of losses.  For large beds, thermal losses from insulated beds are generally very small, typically of the order of 1% of the total useful heat content over a 24 hour period.  I also show that the parasitic loss associated with pumping air through the pebble bed is acceptable for the sort of practical application I have in mind.

Since writing the paper, I have used the simulation code to investigate how pebble bed storage could be used in conjunction with my evaporation engine.    This is the abstract for the conference paper that resulted (Barton, 2012):

A simulation study is presented for air-blown thermal storage in a solar thermal power station powered by passive heat collection under a transparent insulated canopy.  The principal objective of this study is to investigate the round-trip efficiency of thermal storage in a pebble bed.  In the proposed system, heat energy is converted to power by a new heat engine based on evaporative cooling of hot air at reduced pressure.

The work examines the performance of the canopy/engine/storage system over representative days each month for a full year.  The useful heat reclaimed from the storage system is typically about 95% of the useful heat input, less small additional losses at the walls and ducts of the storage system.  Because the heat reclaimed has a smoother daily temperature distribution than the heat gathered by the canopy, there is another 5% penalty in conversion of heat into power.  For the configuration used in this study, the power output using storage is 88% of what would be obtained without storage.  This estimate includes modest losses due to pumping and heat transfer at walls and ducts.  Coarse economic evaluations indicate that storage would reduce the Levelised Cost of Electricity by 27% and increase the Capacity Factor of the engine by 88%.

Conclusion

I’m enthusiastic about the prospects for pebble bed thermal storage.  The pebble bed simulation code will be useful for various applications including dispatchable solar thermal power generation and provision of process heat in domestic and industrial applications.  The next application to be studied will be to my BRRIMS (Brayton-cycle, Re-heated, Recuperated, Integrated, Modular, Storage-equipped) concept for solar thermal power generation.

References

N.G. Barton, “Passive Solar Power Generation with Air-blown Thermal Storage”, Solar2012, Australian Solar Council, Melbourne (2012).

M. Hänchen, S. Brückner, A. Steinfeld, “High-temperature thermal storage using a packed bed of rocks – heat transfer analysis and experimental validation”, Applied Thermal Engineering, 31 (2011) 1798-1806.

Climate change infographic

Just over nine years ago, I resigned my senior job at CSIRO Australia, the national applied research laboratory.  I had spent nearly 30 years as an academic and government researcher in industrial and applied mathematics, and I’d decided that the rest of my productive life would be as an inventor in renewable energy and related fields.  That led to an interesting journey, which still continues.

Most of those nine years have focussed on new ideas for solar thermal power such as BRRIMS.  There have also been investigations of dryers and dehumidifiers and desalination, the latter topic being so secret that I have never described my work in publications or on the internet.

My passion still burns fiercely – I think we are making the world worse, not better, and I want contribute to an improvement.  I am deeply concerned about excessive consumption of resources, especially energy, and the damage this causes to our spaceship home, Planet Earth.  Above all, there is the terrible prospect of long-term climate change that in a worst case might leave the planet uninhabitable for humans.  See here and here for my further comments on these issues.

It’s always a pleasure to meet others holding similar views to mine, especially when they have a different set of competencies, a scientifically honest standpoint, and are willing to promote their views for all to see.

This blog post then is a tribute to the authors of the handful of blogs I read every day.

In the field of climate change, I only read four blogs:

www.skepticalscience.com is, quite simply, astounding.  If my memory serves me well, it was founded by a school teacher who wanted to rebut denialist views of his father.  Or was it the father-in-law?  Whatever, it is now an immensely valuable resource in which up-to-date climate science is presented daily in a very readable way. 

www.realclimate.org is at a higher level.  It’s written by the people who publish in the best journals in the field.  The blog posts aren’t all that frequent, but what appears is of extremely high quality.

www.tamino.wordpress.com is a blog written by “tamino”, nom de plume of a US-based statistician.  He is razor sharp and blogs frequently.  He can be counted on for a lacerating debunking of denialist posts, often within hours of them appearing.

www.moyhu.blogspot.com is written by my former CSIRO colleague Nick Stokes.  A natural genius, Stokes will always find a new way to look at big problems such as climate change and then pursue in-depth practical implementations.

I also follow two blogs concerned with oil and fossil fuel energy:

www.theoildrum.com is a big blog with numerous contributors.  Most of the postings are written by engineers in the fossil fuel industry.  Typical posts deal with technicalities, such as how much oil remains, who is producing what, and what are the major issues that need to be confronted.  The blog is refreshingly free from commercial hype, and I read it to know better the devil that must not be fully exploited.

www.peakoil.net deals with similar issues to The Oil Drum, but is less frequent and has more of scholarly standpoint.

That’s the long and short of the blogs I read.  On some days there is enough reading for 30-45 minutes, on other days I skim the contents in 5-10 minutes.  My daily reading also includes the mainstream media and various e-magazines, but I won’t discuss them here.

I was inspired to write this blog post because I recently had an approach from Allison Lee, who I assume is a young person based in the USA.  She had found my blog and asked if I’d comment on some infographics that she had prepared.  The topic – why it’s climate change of course!  I like the way she assembles relevant information and provides it in a palatable way for a younger audience such as 10-18 year olds at school, or for young adults with similar educational levels.

You can check out Allison’s infographic here.  It might be a useful link for your children or grandchildren.

BRRIMS solar thermal power

Since mid 2012, I’ve been working on a new concept for solar thermal power generation.  I call this BRRIMS, which stands for

• Brayton-cycle
• Re-heated
• Recuperated
• Integrated
• Modular
• Storage-equipped

A provisional patent application has been lodged for the concept.

Today, I’d like to give an overview of BRRIMS.  Let’s start with the flow-sheets.  At the start of day, the recuperator is thermally charged and the stores are discharged.  Here’s the day flow-sheet (click to enlarge diagrams) …

 

 

At the end of the day, the recuperator is discharged and the two stores are thermally charged.  Here’s the flow-sheet for night operations …

 

 

Features to observe are the Brayton-cycle engine with two-stage expansion and intermediate re-heat, the solar collectors (in yellow), the recuperator and the two thermal stores.

 

The solar collectors could be passive (e.g. transparent insulated canopy) or active (e.g. linear Fresnel system with a line focus, or even heliostat/tower).

 

The recuperator and thermal stores consist of pebbles in steel tanks.  The recuperator is charged by the exhaust heat of the engine during night-time operations.  During the day, recuperation of engine exhaust heat diminishes the heating requirement of the collector at the bottom right-hand side of the figures above.

 

The outstanding feature of the recuperated Brayton-cycle engine is its thermal-electric efficiency, as shown in the figure below.  This is for ambient temperature 20°C and collection temperature 250°C.  The recuperated system efficiency is shown as a function of the pressure ratio and the adiabatic efficiency of compression/expansion.  The results imply that a piston-cylinder mechanism is required at 100 kW scale, rather than rotating turbomachinery (for which the adiabatic efficiency of compression/expansion would probably be less than 80%).

 

 

Bearing in mind that compression/expansion losses are the dominant system loss once the heat has been collected, it follows that BRRIMS will have excellent performance at low pressure ratios.  Also, the collection temperature does not need to be particularly high (certainly nowhere near 500°C, which is a typical target temperature for Rankine-cycle systems), so heat collection is relatively cheap and easy.

 

Another important feature is the expected Levelised Cost of Electricity, especially with storage.  If I analyse the LCOE using the same methodology as I’ve used previously on this blog to investigate solar power projects around the world, then the figure below shows the results:

 

The figure includes low and high prices for the Fresnel collectors.  I’ve also shown LCOE estimates for a PV system with battery storage.  My specific assumptions for the PV system are 125 kW, capital cost of AUD2/W and Capacity Factor 0.17.  I’ve used two different prices for battery storage, namely AUD 500 and 1000 per kWhr.  Just in case you think those prices are high, keep in mind that the battery system must have a 25-year lifetime, during which a deep charge/discharge cycle will be carried out almost every day.

With BRRIMS, the LCOE goes down as further storage is implemented.  With PV and battery storage, the LCOE goes up as further storage is implemented. 

The basic module for a BRRIMS system is thought to be around 100 kW, as shown in the concept sketch below:

 

 

The BRRIMS concept has a long list of advantages and benefits:

• thermal storage allows dispatchable power generation
• pebble bed storage is cheap, efficient and good for an infinite number of cycles
• reciprocating engine avoids turbomachinery losses
• re-heat improves power output per cycle
• recuperation gives excellent thermal-electric efficiency
• air is the heat transfer fluid and working gas
• no heat exchangers (other than pebble beds)
• no condensers as in Rankine-cycle engines
• no fans required for heat transfer, no pumps
• fuel co-firing (hybridisation) is possible
• special materials are not required – just steel, glass, mirrors, pebbles, insulation
• only water requirement is for cleaning collectors
• Fresnel collectors available off-the-shelf
• pebble bed storage tanks won’t need elaborate design and can be readily manufactured
• the Levelised Cost of Electricity is expected to be excellent and reduced by storage

 

Further details are available at www.sunoba.com.au.  Detailed information can be made available under Non-Disclosure Agreement.

 

I’m excited about the BRRIMS concept and I’m looking for collaborators and investors to help with the development.